The formation of barrier layers on plastic film is highly desirable to produce a wrapping material for the protection of food products or other articles against deterioration by excluding gas, liquids, vapors, odors and especially water vapor and oxygen, from the package.
The barrier layers can be composed of, for example, aluminum oxide or can contain aluminum oxide.
In commonly assigned copending application Ser. No. 08/258,978 filed Jun. 10, 1994 (now U.S. Pat. No. 5,462,602, issued Oct. 31, 1995) and its divisional application Ser. No. 08/501,532 filed Jul. 12, 1995 (now U.S. Pat. No. 5,571,574), both based upon Italian application RM93A 000385 filed Jun. 11, 1993, there is described a process and apparatus for continuous reactive metal deposition in which a plastic film is coated in a vacuum chamber in a system in which metal is vaporized onto the web and the coating is reacted in a reactive zone to form the metal compound.
In that system a pressure in the deposition zone is lower than the pressure in the vacuum zone so that reactive processes are favored in the latter while deposition is favored in the former. The plastic films can be coated with metal oxides or nitrides in this manner.
Mention may also be made of the commonly assigned copending application Ser. No. 08/222,690 filed Apr. 4, 1994, (U.S. Pat. 5,466,296 of Nov. 14, 1995) and its division Ser. No. 08/415,612 filed Apr. 3, 1995, (now abandoned and replaced by Ser. No. 08/659,028) both based upon Italian application RM93A 000216 filed Apr. 6, 1993 and which disclosed a thin film deposition apparatus capable of use for PECVD and sputtering techniques.
Finally mention should be made of application Ser. No. 08/025,514 filed Mar. 2, 1993, replacing Ser. No. 07/956,736 of Oct. 2, 1992 (now abandoned) which discloses the gradated barrier film of aluminum oxide and silicon oxide which has proved to be highly effective against permeation by water vapor and oxygen and yet has excellent optical properties.
While these systems have been found to be particularly effective and advantageous in the preparation of wrapping films and like damaging materials, further research into the application of barrier layers upon plastic films has shown that earlier PVD or CVD processes which directly deposit oxide coatings on such film are too expensive and too low cost efficiency for widespread use in the preparation of wrapping films with barrier layers against water vapor and oxygen penetration, especially where films of high flexibility are desirable.
While the method described in the commonly-owned Italian application RM93A 000385 and the corresponding U.S. applications has the potential of coating large volumes of film rapidly and with fully oxidized or reacted metal layers, we have found that such layers, especially consisting of aluminum oxide, are not effective as barriers against moisture and oxygen penetration or are less effective as barriers than is desirable. Of course, when combined with silicon oxide as has been described in others of the applications mentioned above, the resulting composite barrier is highly effective against moisture penetration and oxygen penetration, fully oxidized aluminum oxide layers are not as effective to a considerable degree.
Nevertheless aluminum oxide layers are highly desirable for barrier purposes if moisture and oxygen penetrability can be reduced because of the good mechanical correspondence of aluminum oxide layers on plastic film.